todos/App/Sources/View Models/ContentViewModel.swift

import SwiftData
import SwiftUI

extension ContentView {

    /// View model for managing the logic for the content view.
    ///
    /// This observable class encapsulates the state and business logic for the main content view.
    /// It follows the MVVM pattern to separate presentation logic from the view layer.
    ///
    /// The view model manages:
    /// - Presentation state for the new task view within a sheet.
    /// - Selected task group for the navigation split view.
    @Observable
    final class ViewModel {

        // MARK: Properties

        /// Whether the new task view should be shown.
        var newTaskShown: Bool

        /// The currently selected todo task group in the navigation split view.
        ///
        /// When a user selects a group from the sidebar, this property is updated
        /// to reflect the selection. The detail view displays content based on this value.
        var group: TodoGroup?

        // MARK: Initialization

        /// Initializes a new view model instance with default values.
        ///
        /// The view model is initialized with:
        /// - New task view hidden
        /// - No task group selected
        init() {
            newTaskShown = false
            group = nil
        }

        // MARK: Computed

        /// Determines whether the new task button should be shown in the detail view.
        ///
        /// This prevents showing the add button when viewing completed tasks,
        /// as users should not create new tasks in the completed state.
        var showNewTaskButtonInDetails: Bool {
            group != nil
            && group != .completed
        }

        // MARK: Functions

        /// Shows the new task view by setting the presentation state to true.
        ///
        /// This method triggers the presentation of a sheet where users can create a new task.
        /// Called when the floating action button is tapped.
        func showNewTask() {
            newTaskShown = true
        }

        /// Determines whether the new task button should be shown in the sidebar.
        ///
        /// The button appears in the sidebar only on compact horizontal size class devices
        /// (typically iPhones in portrait mode). This provides a way to create tasks when
        /// the sidebar is displayed as a full screen view.
        ///
        /// - Parameter horizontalSizeClass: The current horizontal size class of the environment.
        /// - Returns: `true` if the button should be shown in the sidebar, `false` otherwise.
        func showNewTaskButtonInSidebar(when horizontalSizeClass: UserInterfaceSizeClass?) -> Bool {
            guard let horizontalSizeClass else {
                return false
            }

            return horizontalSizeClass == .compact
        }

    }
}